MInt has a tendency to do that, DD, make itself right at home. Enjoy it!
The recipe is from those long ago days when I was living in Louisville and had Derby parties that included many friends. But a simple syrup is just that, same amounts of water and sugar, so you can pretty well adjust it to one or two drinks fairly easily. It is a good drink, but it packs quite a wallop before you know it.
The cups don't matter much either, they're just for tradition. Same sweet taste in a plastic cup!
It's been a long time since I made more than one julep . . . or two.